Archaic term; in modern Japanese, 同い年 (おないどし) or 同じ年 (おなじとし) is used instead. Found in historical or literary contexts.
In old documents, the expression 'person of the same age' can be found.
Archaic kanji spelling for 'same age'; 庚 is a traditional calendar sign used for age reference.
Variant archaic spelling; 甲 is another calendar sign, but 同庚 is more standard.
Compound of 同 (same) and 庚 (the seventh of the ten celestial stems, used in the traditional Chinese sexagenary cycle to denote years and ages). The exact historical derivation is uncertain, but it follows a pattern of using calendar signs for age-related terms.